Convoy current timing arrangement



July 16, 1940.

u. s. BERGER r-:r A1.

CONVOY CURRENT TIMING ARRANGEMENT Filed July 8, 1959' 4 Sheets-Sheet 1 mm. wm,

u. s. BERGER Er A1. 2,207,711

CONYOY CURRENT TIMING ARRANGEMENT Filed July 8, 1959 July 16, 1940.

4 Sheets-Sheet 4 ik u ik "a TELE GRAPH I NORMAL PEECH -CONVOV CURRENT CONI/0) CURRENT PEEP/Va 75L .ec/MPH l SPEECH CONVOY CURRE/V 7 j as. BERGER Nm/70' ,4.5 Mah/OSE A 77`ORNEV Patented July 16, 1940 MTENT oFFlicE CONVOY CURRENT TIMING ARRANGEMENT Uriah S. Berger, New York, and Alfred E. Melhose, St. Albans, N.V Y., assignors to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application July 8, 1939, Serial N0. 283,369

6 Claims.

This invention relates to communication systems and particularly to systems wherein telegraph signals are interpolated in telephone conversations. The invention is an improvement in 5 the systeml disclosed in application Serial No.

219,964, iiled on'July 19, 1938, by I. E. Cole and A. E. Melhose.r

The object of the invention is to provide a novel circuit arrangement which will insure against mutilation of speech or the intermixture of telephone and telegraph signals. It is important in a system of this nature that the telephone transmission shall approach perfection so that nothing should occur to disturb or annoy a listener. Insurance against disturbing occurrences must be positive and extreme, otherwise the usefulness of the system fails, not in accordance with the frequency of interference but more nearly in totality where such interference first begins to occur. It will, of course, be understood that this is based in large measure on the general expectation of the public that they will be .given as nearly perfect telephone service as it is possible to provide. y

According tothe present invention a channel, including a mutable link or a signaling channel capable of or liable to change from internal or external cause which may give rise to interfering energy or more specifically subject to noise, fading or change of impedance, is employed normally for sending telegraph signals in the form 3f voice frequency alternating current signals. Telephone facilities are also provided at each end of this channel. At the transmitting end there is a voice operated means for switching off the telegraph circuit and switching on the telephone circuit. Also responsive to this switching device there is a means for transmitting over the channel a convoy current in the form of an alternating current of a predetermined and definite frequency just beyond the voice frequency range. Means areprovided at the transmitting end for delaying the voice currents until the convoy current has vbeen transmitted for a period long enough to prevent the occurrence of certain annoying phenomena which will be described presently. At the receiving end, the normal arrangement is a path through a switching point thence through a band-pass filter tuned to the frequency of the alternating current telegraph signals to the telegraph receiving apparatus.v

There is also a branch circuit leading into a bandpass lter tuned to the frequency of the convoy current. When the convoy current is transmitr ted, it goes through this branch path and Causes the operation of the switching means which thereupon cuts off the telegraph receiving circuit and switches on the telephone receiving circuit.

In the operationA of circuits of this nature, certain` annoying phenomena have been noted. First, there is the phenomenon known as clipping. This manifests itself to the listener as the loss of a syllable or a small portion of a syllable at the beginning of each period of conversation. Another 'annoying phenomenon is known as peeping,y so called because it manifests itself to the listener as a short application of telegraph frequency signal at the beginning of each period of conversation.

Clipping is caused by the overlapping in time of the arrival of speech currents at the receiving switchingv point and the operation of the switching means thereat. Peeping is caused by the overlapping in time of the arrival of telegraph signal current at the receiving switching point and the operation of the switching meansthereat. If this switching means is too late in operating clipping will result; if this relay is too early in operating peeping will result. In order to prevent the too late operatng of the switching means sufficient delay of speech currents somewhere between transmission and reception must be providedy to overcome delay in the operation of this means caused bysuch factors as syllabic variation and the adverse eiTect of the mutable link on the yconvoy current after it has passed throughdelaying circuits such as band-pass filters. In order to prevent the too early operation of the receiving switching means, the exact time relation between the telegraph signal and the convoy current must be maintained from the point of transmission into the mutable link up to the receiving switching point. In other words, where i a fading mutable link is the transmitting medium, all the delay must be at the transmitting end because any difference in delay in the different paths traversed by the currents going into the speech path on the one hand and the switching path on the other hand will upset the relation between telegraph signal current and convoy current. If there is any delay at the receiving end, such delay does not affect theoperation of the switching means but does affect the timing of the telegraph signal, and hence the relative effective timing pattern is upset with the result that the switching may take place before thev telegraph signal has ceased to flow to the switching point.

Heretofore various vterms have been used to coil designate that current which is used before or with the transmission of speech to effect the switching at the distant end. In the present case, since this current completely blankets the speech overlapping it at both the head and rear, it may appropriately be termed the convoy current. This is especially so since the relationship of convoy current and speech is maintained in its transmission over the mutable link and through the receiving circuit up to the point where these currents are separated.

The drawings consist of four sheets and contain nine figures.

Fig. 1 is a block diagram illustrating by general schematic the manner in which the present invention operates.

Hg. 2 is a fundamental circuit diagram illustrating prior art.

Fig. 3 is a fundamental circuit diagram illustrating the present invention.

Fig. 4 is a nest of grap-hs showing the form of varying strength signalsat the transmission point.

Fig. 5 is a nest of graphs showing the form of varying strength signals at the receiving point, after they have passed through certain delaying networks, such as band-pass filters.

Fig. 6 is a pair of graphs illustrating the effect on the timing of certain relays caused by the use of band-pass filters.

, Fig. '7 is a fundamental circuit diagram illustrating the invention in more detail than in Fig. 3 above.

- Fig. 8 is a timing chart used to explain the action of the component parts of the invention,

and

Fig. 9 is another chart'illustrating the phenomena of clipping and peeping.

Fig.- 1 is a general schematic of the system of the present invention. A speech path I leads to a hybrid coil Z from which a path 3 for outgoing speech leads to a vogad d. The outgoing speech then passes through a transmitting suppressor 5 and b y means kof an amplifier-detector 6 operates a chain of relays TM (transmitting master relay), TVHO (transmitting-voice hangover relay) and other relays l, herein labeled as other relays of the transmitting switching circuit. This voice operated switching circuit controls a transmitting singing suppressor 9 so that the outgoing speech can pass from the transmitting suppressor 5 through the delay and amplifier 8 and the transmitting singing suppressor 9 `to the hybrid coil I0. From the hybrid coilli the speech reaches another hybrid coil I'I and thence goes through the radio transmitter l2 to an antenna I3. Through radio transmission the speech will be received at some distant point and for purposes of explanation it will be assumed that the distant point has a receiving circuit similar in all respects to the lower part of Fig. l. The speech, therefore, enters antenna ll, .passes through a radio receiver I5 including a detector and a vodas repeater I6 to a hybrid Il. That part of the circuit from radio transmitter I2 to radio receiver I5 is termed a mutable link, since it comprises a signaling channel capable of or liable to change from internal or external cause which may give rise to interfering energy or more specically, subject to noise, fading or change of impedance. From the hybrid coil Il the speech goes into a hybrid coil i8 thence through the receiving singing suppressor IS, the receiving repeater 2Q, a low-pass filter 2l and a path 22 to a hybrid coil, such as 2,

whence it goes to the receiving subscriber over a line such as I.

The voice operated switching circuit 'I controls a convoy current enabler so that the source of current 24 is passed through the convoy current filter 25 to the hybrid coil IIl and thus acompanies speech. The voice operated switching circuit 'I also controls a switching point leading from the receiving switching circuit 26 at the same terminal to the transmitting suppressor 5,

so as to enable this circuit during the period that speech is going out from line I to antenna I3. During this time the voice operated switching circuit l also controls the telegraph control circuit 21 to stop the action of the telegraph transmitting circuit.

At the receiving end the convoy current coming in over antenna Id passes through hybrid coil Il and thence through a telegraph band elimination lter 2S into a convoy current amplier and filter 29. This operates the receiving switching circuit 2B, through a detector .45 and relays CTM (master relay) and RSS (receiving singing suppressor relay). The circuit represented by the rectangle 26 here labeled other relays of the receiving switching circuit performs several functions as follows: one, the receiving switching circuit 26 controls a 4normally open circuit between the telegraph band elimina.-

tion filter 28 and the detector 45, including a speech amplifier 30. This is provided so that when convoy current is first received the receiving switching circuit 26 will be operated thereby and then held operated thereafter by speech currents flowing through the amplifier 3U.

The receiving switching circuit also controls the receiving singing suppressor I9 to open the.

speech path. In practice the 'receiving singing suppressor I9 is operated directly by fthe relay RSSwhich is part of the receiving switching cir-Ik cuit. This switching. circuit also controls a nor-A mally closed path between. thehba'nd-pass filter 3| and the telegrad 32` so that while speechis being received, the telegrad 32 Willbe disabled. At the same time the receiving switching circuit controls the receiving relays for two telegraph channels 33. v

During pauses in the speech either lwhen the subscriber at the distant end is talking intoY channel I or during the time when they subscriber on channel I pauses during his talk over the antenna I3, the telegraph apparatus shown in the Lipper part of the figure is on operation. This comprises perforators 318 and 35 Working, respectively, into tape transmitters 36 and 31 and thence into rotary distributor 38. distributor 38 works through the control circuit 21 which exercises control over the telegraph signal suppressor 39. A source of telegraph signal tone is supplied by the telegraph oscillator dil and through the action of the telegraph signal suppressor 3S under the control of the control Rotary.

II) n they were received over the antenna I 4. These direct current signals then pass through the switching relay circuit 33 and the rotary distributor 4I to the twoprinters 42 and 43. During the reception of telegraph signals the receiving switching circuit 26 is unoperated and the speech path is blocked off by the receiving singing suppressor I9.

A line 44 schematically represents a means for keeping the distributor 38 in exact synchronism with the distant rotary distributor 4I. The rotary distributor 38 and the rotary distributor 4I at the same terminal may or may not be kept in synchronism. In actual practice, they generally are kept in synchronism for reasons having to do with the cost of synchronizing apparatus, but from an operating standpoint it is only necessary that the rotary distributors forming part of the same channel be kept in synchronism.

.In Fig. l. the amplied-detector I3 works into a relay TM., known as the transmitting master. This relay is shown in some detail in order to illustrate the action of the following relay TVHO. When the subscriber on` line i begins to talk, speech currents through the amplifier-detector 6 operate the 'IM relay and close a circuit from ground through `the armature and right contact of relay TM, resistance d6, resistance 4T to the winding of relay TVI-IO and thence to battery. A condenser 43 connected between resistances 4S and 41 comprises a hang-over circuit so that when speech ends and relay TM moves back to normal position, the charging of condenser 48 willmaintain TVI-IO operated for a predetermined period. Relay 'I'VHO therefore does not immediately release the other relays of the transmitting switching circuit but maintains them operated for a short period` known as the hang-over period after speech has ended.

In the same manner and in the receiving circuit the master relay CTM is operated fromy the detector 45 and causes the operation of relay RSS through the resistances 49 and 50 and the condenser 5I. Relay RSS is caused to be slow to release so that another hang-over period is provided at the receiving end to hold the other relays of the receiving switching circuit operated after the convoy current has ceased to operate the master relay CTM.

Going now to the fundamental circuit in Fig. '7, this will be related to the circuit in Fig. l. 52 represents the telegraph sending apparatus in the upper part of Fig. l. 53 represents a switching point for normally connecting `the antenna I3 to the telegraph sender 52. In Fig. l this switching point comprises the action of the transmitting switching circuit 'l through vthe control circuit 2l and through that the control of the telegraph signal suppressor 39. The switching point in its alternate position in Fig. '7 is designed to indicate connection to antenna I3 of the convoy current 24 and the delay 8.. An arrowhead 54 indicates the control of the operation of the switching point 53 by the voice corning from the telephone transmitting point 55 through a rectangle 55, here labeled syllabic variation. This fundamental circuit, therefore, indicates that when voice currents come from transmitter 55 they will operate the switching point 53 thereby cutting olf the telegraph-transmission and connecting instead the convoy current and the voice currents. Due to syllabic variation the operation of the switching point 53 is variable. in its timing, and after the switching point 53 has been operated the voice is delayed by the acoustic delay 8 so that the convoy current Fig. '7. As shown in Fig. l this switching point consists of the control exercised over the line between the band-pass filter 3I and the telegrad 32 and operation of the receiving singing sup-v pressor by the relays of the receiving switching circuit and the control exercised over the receiving singing suppressor I9 by the relays of the receiving switching circuit. The voice currents and telegraph signals, which fall within the voice current range, go over the lower branch to the switching point 5l. With no convo-y current present the circuit isL closed through the band-pass filter 3! to the telegraph receiver here designated 58. When the convoy current is ilowing then the switching point 51 is operated to its alternate contact and closes a path to the telephone receiving apparatus 59.

With the aid of this fundamental circuit diagram it is believed that the further explanation will be somewhat easier to follow.

Fig. 2 shows a fundamental circuit diagram illustrating the arrangement found, for instance, in the Cole-Melhose patent.

In this case speech passes from the transmitting point` 55 to the receiving point 59 through two units of delay, one designated Bil in the transmitting end of the` circuit and the other designated 5I in the receiving end of the circuit. Prior art teachings have been to the eiect that in order to prevent clipping a sumcient amount of delay must'be placed in the speech path so that the control tone will always be able to close the speech path before the speech arrives at the point controlled by such control tone. Heretofore, it was believed and found practical to have this delay at any point in the circuit, either all at one end or the other or divided, part at the transmitting end and part at the receiving end. In the Cole-Melhose patent the delay is shown part at the transmitting terminal and part at the receiving terminal. Experience has shown, however, that asI the mutable link introduces greater variations, such an arrangement is not wholly satisfactory. Experience shows that as a system of this nature is extended, the action of the mutable link becomes a greater factor than might have been Otherwise expected.

Since the various currents at the same time ow in different circuits they encounter diierent delay factors. Speech, for instance, starting with a syllable containing a great amount of energy will cause faster operation of the TVI-IO than will speech starting with a syllable having a small amount of energy. This phenomenon is well vknown and need not further be described. Speech in the present circuit, therefore, is subject to delay but by its own character introduces a delay known as syllabic variation into the operation of both the telegraph and convoy current circuits. At the distant end the band-pass lter throughV which the convoy current and through which the telegraph currents must ow causes delay under the inuence of the mutable link. Due to these various delays, then, the sys'- tem of Fig. 3 has been devised. In this case, two units of delay 6I) and 6I are placed at the trans,- mitting end with the resultr that speech between transmitter 55 and receiver 59 is delayed by the same amount as in the system 0f Fig. 2 but the delay being all at the transmitting end, avoids the phenomenon known as peeping. It is necessary to have the twounits of delay to prevent clipping but these two units must be at the transmitting end to als'o prevent peeping.

Fig. 4 is a nest of graphs showing the envelope of keyed signals, that is, a weak, a medium and a strong signal as plotted between amplitude and time at the point of transmission. In Fig. 5, these same signals, that is, the weak, the medium and the strong are shown at the output of the band-pass filter. This change in form is characteristic and well known. The effect of these is shown in the graphs-in Fig. 6 where two curves are shown plotted between time which it takes to operate a relay, such as the relay RSS against the energy reaching that relay over the mutable link. Given a strong signal, one which might come over the mutable link with very little attenuation, the operate time between the lower curve and the upper curve shows little difference, but, where the signal is weakened by attenuation, the difference of time that it takes the receiving singing suppressor to respond becomes very marked. Thus, with the mutable link introducing variationsI in the strength of signals arriving at the receiving end and operating the receiving singing suppressor relay, the response in this relay will be variable and in accordance with the action of the mutable link.

With this short explanation then the timing scheme of Fig. 1, Fig. 7 and fundamentally of Fig. 3 may be explained by the aid of Fig. 8. This figure consists of a number of horizontal lines each representing the duration in time of a certain factor. Lines A to I, inclusive, represent factors at the transmitting end and lines J to S, inclusive, represent factors at the receiving end, Starting from a given point represented by the vertical line T as the beginning, this timing operation line A represents an interval of speech transmission. This may be a syllable or a number of syllables put together to form a sentence. Lines B and C represent the action of the voice operated switching circuit and particularly the action of a relay therein, known as the transmitting voice hang-over relay TVHO. The amount of time by which these lines extend beyond the limit of the speech transmission A is known as the transmitting hang-over and is determined by the condenser 48. Line B represents the most favorable action of this relay, that is, when speech starts with a syllable having a high energy level. It will be seen that even under the most favorable condition this TVHO relay is delayed very slightly in operating after the speech A has started. The line C represents the most unfavorable action of this relay, that is, when speech starts with a syllable having a low energy level. The difference between the action of this relay under these two conditions is known as syllabic variation.

Lines D and E represent the telegraph signals as they are stopped by the action of the transmitting voice hang-over relay. The line D corresponds to the line B and the line E corresponds to the line C, therefore the difference between lines D and E is also known as syllabic variation. Lignes F and G represent the transmission of convoy current through the action of transmitting voice hang-over relay. Lines F and G correspond to lines B and C, respectively, and Iagain we have an example of syllabic variation. It should be noted that the telegraph signal is cut off at thel instant that the convoy current starts.

Lines H and I represent the speech transmission as it appears at the radio transmitter. Under condition I-I the speech has been delayed one unit of time at the transmitting end as represented by the fundamental circuit, Fig. 2. Under condition I the speech has been delayed two units of time at the transmitting'end as represente by the fundamental circuit, Fig. 3. l v

The four lines J, K, L and lVI represent the action of the receiving switching relays atfthe distant end of the channel and particularly the action of the receiving singing suppressor relay RSS. i Lines J and K show the extreme. operating conditions of the receiving singing suppressor relay under conditions explained in Fig. 6 and these lines correspond to line F showing the most Y favorable transmission of convoy current under syllabic variation. Lines L and M show extreme operating conditions of the receiving singing suppressor relay corresponding to line G they most unfavorable transmission of convoy current under syllabic variation.

The amount of time by which these lines eX- tend beyond the lines F and G is known as the receiving hang-over and is controlled by delay circuit comprising the resistances wand 50 and the condenser 5l.

Lines N and O represent the speech as delivered to the nal receiving circuit. Lines N and vzo O correspond to H and I, respectively, and in' this connection it Will be noted that they both occupy the same final position in time. However, lines I-I and I represent the position in time of the voice under these different-conditions asl they are transmitted over the mutable link and as they appear in the irst hybrid 'coil at the receiving end.

Lines P, Q, R. and S represent the ending ofy telegraph signal tone at the receiving end, P and Q in the case where there is no delay at the receiving end, as in the fundamental circuit, Fig. 3; and R and S in the case where there is delay in that point as in fundamental circuit, Fig. 2. Lines P and R correspond to line D at the transmitting end and lines Q and S correspond to line E. The reason for the extension of lines R. and S well within the time of line A is the fact that the telegraph tone is within the voice range and hence it is delayed in reaching the receiving singing suppressor in the same manner as the voice is delayed.

Now by the amount that line R, overlaps line J and line L or the amount that line S overlaps line K or line M the telegraph tone will be heard by the subscriber beyond the receiving singing suppressor because this apparatus is enabled by that much time before the telegraph signal tone has been cut off. This relation is shown graphically by a regrouping of the pertinent time indications with areas shaded by vertical lines. Such shaded areas represent peeps By the present invention this annoying phenomenon is avoided as shown graphically in the regrouping of lines P and Q and lines J, K, L and M. Here there is no overlapping, in other Words, the telegraph signal has always ended before the receiving singing suppressor has been enabled and hence there is no peeping.

Fig. 9 is in the nature of a rsum and is shown graphically by lines representing time indications in the same manner as in Fig. 8. grouping marked normal it/is noted that the convoy current starts or causes the operation of 75l In the rstr yns reiay'rtss inl between the ending of thel telegraph *signal and the starting y'of speech. This is the normal condition and the one to be desired. [In

YRSS relay has operated then part oi the spreec.L

has been clipped and this `grouping of lines graphically'illustrates the phenomenon of clipping. The third and last` grouping of lines represents peeping Here with delay in that branch of the voice current circuit between antenna le and the switching point l (Fig. 7) the telegraph and speech have been delayed. hence the convoy current has operated the relay RSS before the ending olii' the telegraph, consequently the listener hears a very short interval oi telcgph tone and this phenomenon is known as pc, Ang.

What isclaimed is:

l. In a' communication system, a channel including a mutable link, means for transmitting voice currents over said channel, means for transmitting telegraph vsignals over said channel, means for switching said channel between voice terminal circuits and telegraph terminal circuits, voice operated means at the transmitting end of said channel for transmitting a convoy current over said channel for operating saidv switching means and delay circuits at said transmitting end for timing said convoy current to precede and trail said voice currents vover said mutable link and the receiving end of said channel, the period of time by which said convoy current precedes and trails said voice currents being predetermined to overcome the maximum changed relationship between said convoy current and said voice currents created under extreme conditions in said mutable link whereby switching from telegraph terminal circuit conditions to voice terminal circuit conditions before telegraph transmission inthe receiving end of said channel has ceased is prevented.

2. In a communication system, a'channel int cluding a mutable link, means for transmitting voice currents over said channel, means forA transmitting telegraph signals oversaid channel, means for switching said channel `between voice transmitting circuits and telegraph transmitting v circuits, voice operated meansat the transmitting end of said channel for transmitting a convoy current over said channel for operating said switching means, delay circuits for changing the relationship in time between said voice currents and said accompanying convoy current wherebyA said convoy current is made to overlap said voice currents upon transmission into said mutable link, means in the receiving end of said channel for separating said convoy current and said voice currents, means in that part of the receiving end of said channel into which said convoy currentv is diverted responsive to variations in said mutable link rendering the operation of said switching means at the receiving end of said channel variable in time, and means in said receiving circuit for maintaining they relation in 'fi voice current' responsive means for disabling said telegraphr means, and for ltransmitting 'over said i mutablelink a convoy current, said receiving apparatus comprising meansr responsive to voice "frequency alternating current-telegraph signals, f

means-for diverting voice currents and rsaid con- "voy current intol diiferent paths, means responsive'to said convoy current for-switching said receiving" apparatus from said telegraph means 'to voice current circuits,'and mearisjcomprising voice current delay meansl at the* transmitting end only for adjusting the time relation between the operation of said switching means and the arrival of telegraph signals and voice currents at the switching point in said receiving vapparatus whereby overlapping in time of the operation of said switching means and the presence of either ltelegraph signals or voice currents is prevented.

4. In a communication system, a channel including a mutable link, voice frequency alternating current telegraph signal and telephone transmitting circuits at one end of said channel,

y telegraph and telephone receiving circuits at the other end of said channel, switching means responsive to voice current control at the transmitting end for switching thetransmitting end of said channel from a normal condition in which said telegraph circuits are eiective 'to a condition in which said telephone circuits are eiective, means responsive to saidv switching means for transmitting a convoy current tothe receiving end ci said channel and means thereat reseonsive to convoy current for switching the receiving end of said channel from a normal condition in whichsaid telegraph circuits are efiective to a condition in which said telephone circuits are effective, and means including acoustic delay circuits at the transmitting end` only of said channel for insuring the operation of said switching means at the transmitting end of said channel after the arrival of voice fref quency alternating current telegraph signals at the said receiving end switching point has ceased and Ibefore the arrival of voice currents at said point has begun.

5. In a communication means at the transmittingend of said channel (l) for transmitting voice frequency alternating current telegraph signals over said channel, (2) fortransmitting voice currents over said channel, (3) responsive to voice currents for rendering said telegraph means ineffective and said telephone means eiective, (4) for delaying said voice currents so that there will be a predeterminedtime interval between the ending of telegraph transmission and the start of telephone transmission (5) responsive to voice currents lfor transmitting a convoy current over said channel starting at the ending l of telegraph transmission and ending a predetermined time after the ending of telephone transmission, and means at the receiving end of said channel (l) responsive to said telegraph transmission, (2) responsive to said telephone transmission, (3) responsive to said convoy current for switching said channel from said rst to said second means, (4) for separating said convoy current from said voice currents, and (5) a voice` current rpath from said point of `separation free of voice current delaying means.

6. In a communication system, a channel including a mutable link, means at the transmitting end of said channel (1) for transmitting voice frequency alternating current telegraph signals (2) for transmitting voice currents,

system, a channeh V(3) responsive to voice currents for transmitting a convoy current With said voice currents (4) for vswitching from transmission of telegraph signals to transmission of convoy current and voice currents (5) for delaying Voice currents (6) responsive to syllabic variation for delaying the operation of said switching means, and means at comprising band-pass filters in the path of said l -convoy current for variably delaying the action of said third means under the iniiuence ofv said mutable link (5) comprising band-pass lters in the path of said telegraph signals beyond said switching point, and (6) comprising a path for voice currents and for telegraph signal currents free of delay up to said switchingY point. A

URIAH S. BERGER. ALFRED E. MELI-IOSE. 

